There are a number of different circumstances in which it is desirable to deliver a tightly focused sound beam to a particular location. One useful application is, for example, in advertising to prevent interference and confusion for listeners who hear mixed signals from different broadcasting sources. Higher power sound beams could be used, for example, for public announcements by targeting certain locations or groups of people during demonstrations or open air events. Sound beams with even higher power could be employed to induce physical discomfort in a person or to cause damage in livestock and property.
Hyper-directional sound beams can be produced by driving an acoustic source, such as a single acoustic transducer or an array of acoustic transducers, with a signal consisting of a high-frequency (ultrasonic) carrier wave that is modulated with a low-frequency sound signal. The high-frequency component of the sound wave is absorbed within a short distance from the acoustic source due to self-demodulation on passage through the transmission medium, such as air, leaving only a low-frequency waveform that is related to the modulation signal and that propagates at the speed of sound within the beam defined by the high-frequency signal.
The sound beams produced by the above technique may be focused, steered or projected in a defined area or direction, for example, by rotating or oscillating the acoustic transducer or by making use of digital beam-forming techniques employing phased arrays. The high-frequency audio signal is also not audible prior to demodulation.
While the foregoing arrangements are adequate for a number of applications, there is still a need for a method and system able to efficiently generate a high-power hyper-directional sound beam, and more particularly a sound beam, which produces a time-averaged non-zero sound pressure gradient and a time-averaged non-zero sound pressure force field at a location of a stationary or moving object to exert a net force on the object.